Not Applicable.
As the current trend in offshore oil and gas production advances into deeper waters, it is becoming increasingly necessary for the industry to develop cost effective solutions for developing fields in deep and/or remote waters.
A typical solution for such cases is to keep the production facilities on a xe2x80x9chost platformxe2x80x9d and connect the deep-water well(s) to the platform with pipelines and risers. The supporting equipment for the subsea tree control, such as hydraulic and electric power units, chemical injection pumps and tanks, and a control console, are also housed on the host platform. The subsea tree control is accomplished via long umbilical(s) consisting of electric conductors, hydraulic lines and chemical injection lines laid alongside the pipeline. In addition, two parallel pipelines are necessary to accomplish the roundtrip pigging operations. The distance between the well and the host platform is known as the tieback distance. The cost and technical challenges of this type of conventional tieback system increase as the tieback distance increases, and to a lesser extent as the water depth increases. In most cases, 20 miles represents the practical limit for the maximum tieback distance with the conventional tieback system.
One limit on the length of subsea tiebacks conveying crude petroleum arises from flow assurance problems. Solids such as asphaltene and paraffin deposit on the inner walls of the tiebacks and partially, and in some cases completely, block the flow. The longer the tieback is, the greater the length of pipe that must be inspected and kept free of deposits.
At present, non-intrusive sensors that can adequately detect and characterize such deposits are not available. The present solutions require use of very expensive alternative methods for flow assurance, including twin flowlines (for round-trip pigging), heat traced or insulated tiebacks. These alternative methods operate by attempting to prevent the deposition of solids on the flowline wall, and do not provide means for detecting the presence of solids in the event that deposits occur. The lack of continuous monitoring can result in undesirable shutdowns. For example, a flowline has been kept clear by pigging at a certain frequency, e.g. once per month, and the composition of the fluid in the flowline changes so that deposits begin to form at a greater rate, the line will become clogged and possible shut down because the previously established pigging frequency is now insufficient.
Guided acoustic waves similar those described in U.S. Pat. No. 5,892,162, have been used to detect corrosion in pipes based on reflections from corroded regions. Corrosion and scaling has also been detected in insulated pipelines on surface using guided waves and literature regarding this has been published from Imperial College, University of London.
Monitoring devices such as that described in U.S. Pat. No. 4,490,679 identify paraffin by monitoring change in the resistance of an electromagnetic coil. The monitoring device requires access to the fluid and is housed in a recess in the pipe. It is desired to provide monitoring without disrupting the flow of fluid through the line and without requiring direct contact with the fluid.
In U.S. Pat. No. 4,483,247, an optical asphaltene sensor is described. This sensor determines the content of asphaltene in heavy oils, based on the absorption spectra of asphaltene. The invention uses visible light in the region 500 nm to 1000 nm and thus requires at least optical access to the fluid. Furthermore, it does not distinguish between deposited and suspended asphaltene solids.
Similarly, ultrasonic longitudinal wave measurements have been used to characterize fluids using reflectance methods, as in U.S. Pat. No. 4,571,693. Shear reflectance has been used in prior art to monitor casting processes as in U.S. Pat. No. 5,951,163, detect viscosity as in U.S. Pat. No. 3,903,732, or density as in U.S. Pat. No. 5,886,250 and to monitor the rheology of fluids.
Hence, it is desired to provide a system that can operate over greater tieback distances without the cost and technical disadvantages that heretofore have prevented increasing the tieback distance. It is further disired to provide a method and apparatus for detecting and characterizing deposits of asphaltene, paraffin or hydrates on the inside wall of a pipeline. It is further desired to provide a system that can be installed on a conventional pipeline and does not impede the flow of fluid through the pipeline. The desired system should be able to compensate for drift in the response of its components and should be capable of operating for a period of years without service or calibration.
The present invention provides a method and apparatus that allows non-invasive monitoring of longer tieback distances without the cost and technical disadvantages associated with previous methods. The system of the present invention measures the acoustic properties of deposits on the inner surface of the pipe wall. One object of the invention is to detect, characterize and determine the extent of deposition and thus enable remedial procedures.
The present system detects deposits or deposition of asphaltene and paraffin on the inside wall of a pipeline without impeding the flow of fluid through the pipeline. Furthermore, the present system compensates for drift in the response of its components and is therefore capable of operating for period of years without service or calibration.
In particular, the present system includes an acoustic sensor that is capable of detecting and characterizing deposits of paraffin, asphaltene or hydrates on the inner walls of pipes, thus enabling timely intervention and flow assurance. In one embodiment, the sensor detects and monitors deposition in a section of the pipe. In another embodiment, multiple installations of the system allow the location of depositions to be determined with a desired degree of precision.
The present apparatus is capable of self-calibration and is not affected by drifts in equipment response that may be caused by variations in temperature or pressure or by the passage of time. The present sensors distinguish between types of deposition material based on the frequency and phase response.
In one embodiment, the present system is used to monitor and characterize the deposition and build-up of materials such as paraffin, asphaltene and hydrates in subsea tiebacks. Alternatively, the present system can be permanently installed in a borehole to monitor deposition therein. The present sensor can also be used on surface pipelines to monitor deposition of solids in cases where solids deposition may occur, such as multiphase flow.
In a preferred embodiment, the sensor distinguishes the type of deposition material based on the compression and shear impedance as well as signal arrival times.